Do you see what I see?

We take for granted the world exists as we see it, but centuries of philosophical and scientific debate suggest it might all, quite literally, be in your head. So is reality really what you see, or just an elaborate illusion?

What and how we see has been debated for well over two thousand years, with Greek philosophy offering some of the earliest recorded studies. One popular theory during the fourth century BC, supported by the Pythagoreans, was ‘extramission’: the idea that the eye emanated a light that beamed outward towards objects, illuminating them like a torch. In opposition to this was ‘intromission’, supported by the Greek Atomists including Aristotle, which believed that the world transferred its likeness (eidola) into the eye. This argument was reinforced by the fact you can see the world reflected on the lens of the eye, as though it's been captured inside.

Anatomy of the eye, Margarita philosophica, Gregor Reisch c.1503

Taken literally, these theories seem ridiculous to us now, so it’s perhaps surprising that such beliefs, particularly extramission, supported by Galen in the second century AD, would influence our conception about vision right up to the seventeenth century. By then the physical structure of the eye had been dissected and investigated in all manner of ways, and its anatomy categorised, if not completely understood. The advanced theories of Middle Eastern scholars such as Alhazen and Al-Biruni, writing during the Islamic Golden Age, had also been translated from Arabic into Latin from the eleventh through to the thirteenth century, making their extensive discoveries available in the west.

On the eye and vision, Leonardo da Vinci, c.1504-1506.

Technological advances during the Renaissance, namely the development of linear perspective in the arts, the study of the camera obscuraandthe investigation of optical lenses, helped to improve understanding. But how all the component parts actually worked to produce perception remained a mystery.

It wasn’t until 1604, with the publication of Johannes Kepler’s Ad Vitellionem Paralipomena, that a truly modern theory of vision was finally described. Kepler’s studies advanced the investigation into vision, specifically addressing why, when optical images are flat and inverted, we see things the right way up and in three-dimensions. The only sensible answer to this conundrum was the influence of the brain.

Diagram of ocular refraction, Rene Descartes, La Dioptrique, 1637.

This role of cognition in vision was explored by Rene Descartes, whose epistemological investigations contributed much to both art and science. In La Dioptrique, published in 1637, Descartes concerned himself with the study of light and optics, arguing that our conception of ‘very perfect images’ received by the retina could only be realised in the mind. According to Descartes, our knowledge of the world was completely untrustworthy, a mental representation constructed through what he called ‘sense datum’ that may (or may not) look like the actual thing we see. This stored data is, he argued, what we reference when we dream or hallucinate, like a vast library of visual knowledge full of things we classify as 'familiar'. After Descartes, knowing could never again be separated from seeing.

Helmholtz's ophthalmoscope

The mid-nineteenth century saw the advancement of Kepler and Descartes’ theories by integrating the physics and physiology of vision into a visual psychology. It was the constructivist Hermann von Helmholtz, the 'reinventor' of the ophthalmoscope in 1851, who combined the two in his Handbook of Physiological Optics (1867), furthering the argument that reality could never be directly perceived as it was bound to be mediated by the brain.

We used to think the Earth is flat, because it looks that way. Pythagorus discovered that we were wrong. Then we thought that the Earth is the unmoving center of the Universe, again because it looks that way. Copernicus and Galileo discovered, again, that we were wrong.

Fast-forward to the present and we discover that Descartes and Helmholtz’s theories continue to influence and inform philosophical and scientific thought. In 2013 the neuroscientist Dr Melvyn Goodale, Director of the Brain and Mind Institute at the University of Western Ontario, said that our perception of the world was ‘not slavishly driven by the pattern of light on the eye but is also shaped by our memories, emotions and expectations. Perception and memory literally blend into one another.’ And in a 2015 TED talk Donald Hoffman, Professor of Cognitive Science at the University of California, called the relationship between our brain and our conscious experiences ‘the greatest unsolved mystery in science’, arguing that we can't trust the way things look to represent their true nature because they exist completely outside of our consciousness. Instead, we reconstruct the world as we think it is, shaped by our inferences and assumptions. Why else would so much of the brain’s function be busily engaged in perception at any one time? Hoffman says that perception is an interface that ‘hides reality’ – a user interface that allows us access to a world that is just too complicated for us to comprehend ‘as it is.’ Sound familiar?

The rotating mask illusion, first demonstrated by psychologist Richard Gregory, perfectly illustrates why we can't trust our eyes. As the mask spins around to reveal the inside we should be able to see its features in a concave format. Instead, it transforms almost magically into a protruding three-dimensional face. Our brain can't see the inside of the mask ‘as it is’ because we are conditioned to recognise a face in a particular way. Many other optical illusions prove that what we perceive is not always what is there.

Of course, there are many arguments to the contrary and supporters of direct realism would strongly oppose the position taken by Descartes and others mentioned here. But what is clear is that even in the twenty-first century there remains an ambiguous relationship between the visual and the visualised - an unknown area yet to be discovered - which begs the question, do you see what I see?